Interpretive Summary: Cattle contain several subsets of T cells. The different role of these T cell subsets in the immune response to Bovine Respiratory Syncytial Virus (BRSV) infection is unknown, as is the role of vitamin D in their response. Bovine T cell subsets were purified from normal and BRSV infected calves and stimulated with viral products in the presence or absence of vitamin D. The T cells responded to viral products and virus infection, and there was a difference in the role of individual subsets in the immune response. Vitamin D did not alter these responses. Additional research is needed to understand the role of individual T cell subsets in the bovine.

Technical Abstract:
Gamma delta T cells have recently been shown to respond to stimulation via toll like receptors (TLR). Bovine gamma delta T cells express TLR3 and TLR7, endosomal receptors that are key for the recognition of viruses such as bovine respiratory syncytial virus (BRSV); however, responses of gamma delta T cells to stimulation via these receptors, as well as their role during viral infections, remains unclear. Here, we demonstrate that neonatal bovine gamma delta T cell subsets exhibit robust chemokine and cytokine production in response to the TLR3 agonist, Poly(I:C), and the TLR7 agonist, Imiquimod. Importantly, we observe a similar phenotype in gamma delta T cell subsets purified from calves infected with BRSV. Bovine gamma delta T cells can be divided into subsets based upon their expression of the scavenger receptor WC1, and the response to TLR stimulation and viral infection differs between these subsets. We further report that the active vitamin D metabolite 1,25- dihydroxyvitamin D3 does not alter gamma delta T cell responses to viral TLR agonists or BRSV. To our knowledge, this is the first characterization of the gamma delta T cell response during an in vivo BRSV infection and the first suggestion that gamma delta T cell subsets contribute to the recruitment of inflammatory populations during viral infection. Based on our results, we propose that circulating gamma delta T cells are poised to rapidly respond to viral infection and suggest an important role for gamma delta T cells in the innate immune response of the bovine neonate.